Phase Compensation in Fractional-N PLL and DDS Frequency Generation
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Solution Overview
Problem
Existing Software Defined Radio (SDR) receivers face challenges in achieving continuous signal quality and rapid frequency transitions across a broad frequency range due to the slow slew rate and limited frequency range of Fractional-N PLL synthesizers, which result in nondeterministic signal quality during frequency changes.
Innovation Solution
A frequency generation unit comprising a fractional-N synthesizer, a voltage controlled oscillator, and a direct digital synthesizer, where the combination remains perpetually locked, allowing for almost instantaneous frequency changes across a broad band using a single VCO, eliminating nondeterministic signal quality during transitions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Fractional-N PLL synthesizer is used to achieve continuous signal quality, then spurious performance is improved (in excess of -80 dBc), but frequency transition speed deteriorates (long transition time of indeterminate phase)
Solution Approach 1:
The system is divided into two independent frequency generation paths: a Fractional-N PLL synthesizer path for maintaining signal quality, and a DDS path for enabling rapid frequency transitions. Each path operates independently but contributes to the same output, allowing the system to achieve both continuous signal quality and fast frequency changes without compromising either function
Solution Approach 2:
The patent combines the output signals from the Fractional-N PLL synthesizer and the DDS into a single output signal. This merging allows the system to leverage the signal quality advantages of the PLL while simultaneously utilizing the fast frequency transition capabilities of the DDS, resolving the contradiction between signal quality and transition speed
2Reliability
If a Fractional-N PLL synthesizer with discrete resonator VCO is used, then signal quality is improved, but frequency range adaptability deteriorates (limited output frequency range of less than 20% or +/â10% about the frequency of the signal output from the VCO)
Solution Approach 1:
The system makes a single VCO serve multiple functions: it operates in a locked condition with the Fractional-N PLL synthesizer for signal quality, and simultaneously provides frequency tuning capability across a broad range through the DDS. The VCO is no longer limited to a narrow frequency range but can support both high-quality signal generation and broad frequency adaptability through its dual role in the combined architecture
3Adaptability or versatility
If frequency transition is performed in a Fractional-N PLL synthesizer, then frequency flexibility is improved, but signal quality deteriorates (nondeterministic signal quality with unknown frequency during transition)
Solution Approach 1:
The DDS is pre-configured with the target frequency information before the PLL completes its transition. This allows the system to prepare the frequency change in advance through the DDS path, which can immediately jump to the new frequency while the PLL transitions in the background, ensuring continuous signal quality without nondeterministic periods
Solution Approach 2:
The DDS acts as an intermediary during frequency transitions, providing a clean, deterministic frequency path while the PLL transitions in the background. The DDS output serves as a temporary mediator that maintains signal quality during the PLL's transition period, preventing the nondeterministic signal quality that would otherwise occur
Data Source
AI summary
A frequency generation unit is provided that permits a receiver to tune from channel to channel without cycle skipping and in which compensation for phase offset introduced during tuning is provided. The frequency generation unit includes a fractional-N synthesizer, a voltage controlled oscillator (VCO), and a direct digital synthesizer (DDS). The fractional-N synthesizer generates frequencies from the VCO as well as a temperature controlled crystal oscillator. Outputs from the fractional-N synthesizer are supplied both the VCO and the DDS to control the VCO and DDS. The combination of the voltage controlled oscillator and fractional-N synthesizer is perpetually locked. The fractional-N synthesizer is maintained in a locked condition. The VCO output is provided to the DDS. An output from the DDS or from the fractional-N synthesizer forms the output signal of the frequency generation unit.